Closed-Loop Active Control of Combustion Instabilities Using Subharmonic Secondary Fuel Injection

The results of a study of closed-loop active control of combustion instabilities using subharmonic secondary fuel injection are presented. The tests were conducted in a laboratory-scale dump combustor at atmospheric pressure withaninlettemperatureof 415 ± C,aninletbulkvelocity of 9.3 m/s,andovera rangeoffuel-lean equivalenceratios from lean blowout to the stoichiometric condition. The observed instability was a longitudinal mode instability with a frequency of » 350 Hz. Control was achieved using modulated secondary fuel injection on every fourth cycle of the unstable e ame. The injection of secondary fuel was phase-locked with the pressure oscillation in the combustor, where the relative phase was adjusted to achieve maximum control. The most effective control was achieved at an equivalence ratio of 0.85, where the combustion-generated noise level was reduced by 22 dB, with a secondary fuel e ow rate that was only 2% of the primary fuel e ow rate.